Ibotenic acid lesions in the pedunculopontine region result in recovery of visual orienting in the hemianopic cat

The subcortical and neurochemical organization of the ventral and dorsal attention networks

Attention is a core cognitive function that filters and selects behaviourally relevant information in the environment. The cortical mapping of attentional systems identified two segregated networks that mediate stimulus-driven and goal-driven processes, the Ventral and the Dorsal Attention Networks (VAN, DAN). Deep brain electrophysiological recordings, behavioral data from phylogenetic distant species, and observations from human brain pathologies challenge purely corticocentric models. Here, we used advanced methods of functional alignment applied to resting-state functional connectivity analyses to map the subcortical architecture of the Ventral and Dorsal Attention Networks. Our investigations revealed the involvement of the pulvinar, the superior colliculi, the head of caudate nuclei, and a cluster of brainstem nuclei relevant to both networks. These nuclei are densely connected structural network hubs, as revealed by diffusion-weighted imaging tractography. Their projections establish interrelations with the acetylcholine nicotinic receptor as well as dopamine and serotonin transporters, as demonstrated in a spatial correlation analysis with a normative atlas of neurotransmitter systems. This convergence of functional, structural, and neurochemical evidence provides a comprehensive framework to understand the neural basis of attention across different species and brain diseases.

Association Cortex Is Essential to Reverse Hemianopia by Multisensory Training

Hemianopia induced by unilateral visual cortex lesions can be resolved by repeatedly exposing the blinded hemifield to auditory-visual stimuli. This rehabilitative "training" paradigm depends on mechanisms of multisensory plasticity that restore the lost visual responsiveness of multisensory neurons in the ipsilesional superior colliculus (SC) so that they can once again support vision in the blinded hemifield. These changes are thought to operate via the convergent visual and auditory signals relayed to the SC from association cortex (the anterior ectosylvian sulcus [AES], in cat). The present study tested this assumption by cryogenically deactivating ipsilesional AES in hemianopic, anesthetized cats during weekly multisensory training sessions. No signs of visual recovery were evident in this condition, even after providing animals with up to twice the number of training sessions required for effective rehabilitation. Subsequent training under the same conditions, but with AES active, reversed the hemianopia within the normal timeframe. These results indicate that the corticotectal circuit that is normally engaged in SC multisensory plasticity has to be operational for the brain to use visual-auditory experience to resolve hemianopia.

Using superior colliculus principles of multisensory integration to reverse hemianopia

The diversity of our senses conveys many advantages; it enables them to compensate for one another when needed, and the information they provide about a common event can be integrated to facilitate its processing and, ultimately, adaptive responses. These cooperative interactions are produced by multisensory neurons. A well-studied model in this context is the multisensory neuron in the output layers of the superior colliculus (SC). These neurons integrate and amplify their cross-modal (e.g., visual-auditory) inputs, thereby enhancing the physiological salience of the initiating event and the probability that it will elicit SC-mediated detection, localization, and orientation behavior. Repeated experience with the same visual-auditory stimulus can also increase the neuron's sensitivity to these individual inputs. This observation raised the possibility that such plasticity could be engaged to restore visual responsiveness when compromised. For example, unilateral lesions of visual cortex compromise the visual responsiveness of neurons in the multisensory output layers of the ipsilesional SC and produces profound contralesional blindness (hemianopia). The possibility that multisensory plasticity could restore the visual responses of these neurons, and reverse blindness, was tested in the cat model of hemianopia. Hemianopic subjects were repeatedly presented with spatiotemporally congruent visual-auditory stimulus pairs in the blinded hemifield on a daily or weekly basis. After several weeks of this multisensory exposure paradigm, visual responsiveness was restored in SC neurons and behavioral responses were elicited by visual stimuli in the previously blind hemifield. The constraints on the effectiveness of this procedure proved to be the same as those constraining SC multisensory plasticity: whereas repetitions of a congruent visual-auditory stimulus was highly effective, neither exposure to its individual component stimuli, nor to these stimuli in non-congruent configurations was effective. The restored visual responsiveness proved to be robust, highly competitive with that in the intact hemifield, and sufficient to support visual discrimination.

Perturbation-driven paradoxical facilitation of visuo-spatial function: Revisiting the 'Sprague effect'

The 'Sprague Effect' described in the seminal paper of James Sprague (Science 153:1544-1547, 1966a) is an unexpected paradoxical effect in which a second brain lesion reversed functional deficits induced by an earlier lesion. It was observed initially in the cat where severe and permanent contralateral visually guided attentional deficits generated by the ablation of large areas of the visual cortex were reversed by the subsequent removal of the superior colliculus (SC) opposite to the cortical lesion or by the splitting of the collicular commissure. Physiologically, this effect has been explained in several ways-most notably by the reduction of the functional inhibition of the ipsilateral SC by the contralateral SC, and the restoration of normal interactions between cortical and midbrain structures after ablation. In the present review, we aim at reappraising the 'Sprague Effect' by critically analyzing studies that have been conducted in the feline and human brain. Moreover, we assess applications of the 'Sprague Effect' in the rehabilitation of visually guided attentional impairments by using non-invasive therapeutic approaches such as transcranial magnetic stimulation (TMS) and transcranial direct-current stimulation (tDCS). We also review theoretical models of the effect that emphasize the inhibition and balancing between the two hemispheres and show implications for lesion inference approaches. Last, we critically review whether the resulting inter-hemispheric rivalry theories lead toward an efficient rehabilitation of stroke in humans. We conclude by emphasizing key challenges in the field of 'Sprague Effect' applications in order to design better therapies for brain-damaged patients.

Animal Models of Cerebral Neglect and Its Cancellation

The purpose of this perspective is twofold: 1) to alert and inform the neurospychology and neurology communities on how animal models can improve our understanding of spatial neglect in humans, and 2) to serve as a guide to rehabilitation strategies. Spatial neglect is a neurological syndrome that is inextricably linked to the ability to overtly or covertly reorient attention to new loci. Literature describing variants of neglect leads to the perception of lesion-induced neglect as a uniquely human syndrome for which there are limited treatment options. To the contrary, neglect has been reversed in laboratory animals, and results show that adequate neural representations and motor mechanisms for reversal are present despite damaged or deactivated cerebral cortex. These results and conclusions provoke thought on strategies that can be employed on humans to cancel neglect, and they suggest that long-term amelioration of neglect can be induced by training of specific bypass circuits.

Multisensory stimulation in hemianopic patients boosts orienting responses to the hemianopic field and reduces attentional resources to the intact field

PURPOSE

Lateralised lesions can disrupt inhibitory cross-callosal fibres which maintain interhemispheric equilibrium in attention networks, with a consequent attentional bias towards the ipsilesional field. Some evidence of this imbalance has also been found in hemianopic patients (Tant et al., 2002). The aim of the present study was to reduce this attentional bias in hemianopic patients by using multisensory stimulation capable of activating subcortical structures responsible for orienting attention, such as the superior colliculus.

METHODS

Eight hemianopic patients underwent a course of multisensory stimulation treatment for two weeks and their behavioural and electrophysiological performance was tested at three time intervals: baseline 1 (before treatment), control baseline 2 (two weeks after baseline 1 and immediately before treatment as a control for practice effects) and finally after treatment.

RESULTS

The results show improvements on various clinical measures, on orienting responses in the hemianopic field, and a reduction of electrophysiological activity (P3 amplitude) in response to stimuli presented in the intact visual field.

CONCLUSIONS

These results suggest that the primary visual deficit in hemianopic patients might be accompanied by an ipsilesional attentional bias which might be reduced by multisensory stimulation.

Sensory Cortical Control of a Visually Induced Arrest Behavior via Corticotectal Projections

Innate defense behaviors (IDBs) evoked by threatening sensory stimuli are essential for animal survival. Although subcortical circuits are implicated in IDBs, it remains largely unclear whether sensory cortex modulates IDBs and what the underlying neural pathways are. Here, we show that optogenetic silencing of corticotectal projections from layer 5 (L5) of the mouse primary visual cortex (V1) to the superior colliculus (SC) significantly reduces an SC-dependent innate behavior (i.e., temporary suspension of locomotion upon a sudden flash of light as short as milliseconds). Surprisingly, optogenetic activation of SC-projecting neurons in V1 or their axon terminals in SC sufficiently elicits the behavior, in contrast to other major L5 corticofugal projections. Thus, via the same corticofugal projection, visual cortex not only modulates the light-induced arrest behavior, but also can directly drive the behavior. Our results suggest that sensory cortex may play a previously unrecognized role in the top-down initiation of sensory-motor behaviors.

[Superior colliculus as a subcortical center for visual selection]

Our visual system has limited resources, which need to be allocated in priority to the most relevant elements of the environment. The brain centers of this allocation mechanism, called visual attention, have been studied primarily in cortex. In this review, we describe the role of the superior colliculus, a structure of the brainstem, in attention control. This nucleus exerts its influence on visual selection independently of cortical attentional mechanisms. The exact nature of the subcortical circuits involved remains unknown but it can be hypothesized that the loop connecting the superior colliculus to the basal ganglia are a central actor of this subcortical selection process.

Multiple sessions of transcranial direct current stimulation to the intact hemisphere improves visual function after unilateral ablation of visual cortex

Damage to cerebral systems is frequently followed by the emergence of compensatory mechanisms, which serve to reduce the effects of brain damage and allow recovery of function. Intrinsic recovery, however, is rarely complete. Non-invasive brain stimulation technologies have the potential to actively shape neural circuits and enhance recovery from brain damage. In this study, a stable deficit for detecting and orienting to visual stimuli presented in the contralesional visual hemifield was generated by producing unilateral brain damage of the right posterior parietal and contiguous visual cortical areas. A long regimen of inhibitory non-invasive transcranial direct-current stimulation (cathodal tDCS, 2 mA, 20 min) was applied to the contralateral (intact) posterior parietal cortex over 14 weeks (total of 70 sessions, one per day, 5 days per week) and behavioral outcomes were periodically assessed. In three out of four stimulated cats, lasting recovery of visuospatial function was observed. Recovery started after 2-3 weeks of stimulation, and recovered targets were located first in the periphery, and moved to more central visual field locations with the accrual of stimulation sessions. Recovery for moving tasks followed a biphasic pattern before reaching plateau levels. Recovery did not occur for more difficult visual tasks. These findings highlight the ability of multiple sessions of transcranial direct-current stimulation to produce recovery of visuospatial function after unilateral brain damage.

The efference cascade, consciousness, and its self: naturalizing the first person pivot of action control

The 20 billion neurons of the neocortex have a mere hundred thousand motor neurons by which to express cortical contents in overt behavior. Implemented through a staggered cortical "efference cascade" originating in the descending axons of layer five pyramidal cells throughout the neocortical expanse, this steep convergence accomplishes final integration for action of cortical information through a system of interconnected subcortical way stations. Coherent and effective action control requires the inclusion of a continually updated joint "global best estimate" of current sensory, motivational, and motor circumstances in this process. I have previously proposed that this running best estimate is extracted from cortical probabilistic preliminaries by a subcortical neural "reality model" implementing our conscious sensory phenomenology. As such it must exhibit first person perspectival organization, suggested to derive from formating requirements of the brain's subsystem for gaze control, with the superior colliculus at its base. Gaze movements provide the leading edge of behavior by capturing targets of engagement prior to contact. The rotation-based geometry of directional gaze movements places their implicit origin inside the head, a location recoverable by cortical probabilistic source reconstruction from the rampant primary sensory variance generated by the incessant play of collicularly triggered gaze movements. At the interface between cortex and colliculus lies the dorsal pulvinar. Its unique long-range inhibitory circuitry may precipitate the brain's global best estimate of its momentary circumstances through multiple constraint satisfaction across its afferents from numerous cortical areas and colliculus. As phenomenal content of our sensory awareness, such a global best estimate would exhibit perspectival organization centered on a purely implicit first person origin, inherently incapable of appearing as a phenomenal content of the sensory space it serves.

Superior colliculus and visual spatial attention

The superior colliculus (SC) has long been known to be part of the network of brain areas involved in spatial attention, but recent findings have dramatically refined our understanding of its functional role. The SC both implements the motor consequences of attention and plays a crucial role in the process of target selection that precedes movement. Moreover, even in the absence of overt orienting movements, SC activity is related to shifts of covert attention and is necessary for the normal control of spatial attention during perceptual judgments. The neuronal circuits that link the SC to spatial attention may include attention-related areas of the cerebral cortex, but recent results show that the SC's contribution involves mechanisms that operate independently of the established signatures of attention in visual cortex. These findings raise new issues and suggest novel possibilities for understanding the brain mechanisms that enable spatial attention.

Recovery of function following unilateral damage to visuoparietal cortex

Damage to the visuoparietal cortex located in the banks of the middle suprasylvian gyrus of the cat has been shown to produce a deficit in the detection and localization of moving visual cues presented in the contralesional visual hemifield. There is evidence from reversible cooling deactivation studies that the integrity of this orienting function is not completely dependent on the VP cortex and that under the right circumstances, other brain regions may come online and completely take over the processing that subserves this behavior. We examined the recovery of orienting behavior after unilateral damage to the VP cortex. We found that consistent with previous data, VP damage produced an impairment in the capacity to detect and orient to moving visual stimuli in the contralesional visual field. Over a span of days, spontaneous recovery fully occurred. The ability to detect and localize static visual stimuli was tested as a fiducial measure of parietal cortex function, and this function did not recover. We conclude that the detection and localization of moving visual stimuli is not a function that requires VP cortex and argue for the existence of a parallel and redundant subcortical-cortical brain network that serves as the substrate for recovery of function.

Periparabigeminal and adjoining mesencephalic tegmental field projections to the dorsolateral periaqueductal grey in cat - a possible role for oculomotor input in the defensive system

The dorsolateral column of the mesencephalic periaqueductal grey (PAGdl) differs from its adjacent columns in terms of afferent and efferent connections and the distribution pattern of different histochemical substances. Functionally, PAGdl is associated with aversive and defensive behaviours, but in an earlier study of this laboratory [E.M. Klop et al. (2005) J. Comp. Neurol., 492, 303-322], it was found that PAGdl specifically receives input from the nucleus prepositus hypoglossi, which plays a role in oculomotor control. In search for other oculomotor-related brainstem structures projecting to PAGdl we studied the projections from the parabigeminal nucleus (PBGN) and its medially adjoining periparabigeminal area (PPBGA). In three cats, injections of wheatgerm agglutinin-horseradish peroxidase involving PAGdl did not, or to only a very limited extent, result in retrogradely labelled neurons in PBGN. When the peripheral parts of PAGdl were involved in the injection site, labelled neurons were located in PPBGA, while after an injection involving only the more central parts of PAGdl they were located in the tegmentum medial to the PPBGA. An anterograde tracing study using [3H]-leucine and biotinylated dextran amine affirmed that neurons in PPBGA project to more peripheral parts of PAGdl, while neurons located in the tegmentum medial to PPBGA project mainly to its central parts. These results provide further evidence for the existence of two different subdivisions of PAGdl. We hypothesize that PAGdl is alerted by sudden changes in the visual field, and that the PAGdl defensive system is inhibited when these changes are caused by eye movements.

Subcortical modulation of spatial attention including evidence that the Sprague effect extends to man

The Sprague effect is well-established-small tectal lesions restore visual orientation in the hemianopic field of animals with extensive unilateral geniculo-striate lesions. Studies of human midbrain visual functions are rare. This man with a midbrain tumour developed left-neglect through subsequent right frontal damage. Bilateral orientation returned after clear evidence of damage to the superior colliculus contralateral to the cortical lesion (showing the Sprague effect extends to man). Sustained right-neglect developed after probable additional damage to right superior colliculus. The regulation of spatial attention by tecto-pulvinar circuits is discussed, and it is argued that the reduced right tecto-pulvinar activity (consequent to the additional right collicular damage) was offset by over-compensatory increase in thalamic reticular nucleus (TRN) suppression of left pulvinar activity.

Functional circuitry underlying natural and interventional cancellation of visual neglect

A large body of work demonstrates that lesions at multiple levels of the visual system induce neglect of stimuli in the contralesional visual field and that the neglect dissipates as neural compensations naturally emerge. Other studies show that interventional manipulations of cerebral cortex, superior colliculus or deep-lying midbrain structures have the power to attenuate, or cancel, the neglect and reinstate orienting into a neglected hemifield, and even into a profound cortically blind field. These results, and those derived from experiments on the behavioral impacts of unilateral and bilateral lesions, lead us to evaluate the repercussions of unilateral and bilateral deactivations, neural compensations and cancellations of attentional deficits in terms of an overarching hypothesis of neglect. The cancellations can be both striking and enduring, and they suggest that therapeutic strategies can be developed to reverse or ameliorate neglect in human patients. Animal studies show that in many instances of neglect adequate representations and the accompanying motor mechanisms are present despite the lesion and they simply need to be unmasked and brought into use to effect a remedy.